The present invention relates to pulping digesters (or reactors) for delignifying wood chips to produce wood pulp, and more particularly, to an apparatus and method for determining the height of the mass of chips dispersed in a digesting medium present in the digester.
Conventional pulping digesters are cylindrical vessels, having a height on the order of two hundred feet and a diameter on the order of sixteen feet. A digesting medium, normally referred to as "liquor," and wood chips are introduced into the top of the digester while liquor and wood pulp are withdrawn from the bottom. As the chips travel through the digester from top to bottom, the lignin in the chips is chemically broken down to free the cellulosic material in the chips and to form a pulp suitable for paper making and other uses. The digester is normally maintained in a condition in which it is full of liquor. As chips are continuously fed into the digester from the top, the chips tend to settle in the liquor, leaving a region at the top of the digester containing substantially chip-free liquor. Thus, under normal operating conditions, the level of chips in the digester is below the top of the digester, while the digester is full of liquor.
The throughput of a pulping digester, although sometimes automatically controlled with the aid of a computer, is often manually controlled by an operator. The operator controls the chip level by increasing or reducing the rate at which chips and liquor are introduced into the digester and by increasing or reducing the rate at which pulp and liquor are withdrawn from the digester. When more chips and liquor are added to the digester than pulp and liquor withdrawn, the chip level will rise, and vice versa. If the operator has no accurate or convenient means for determining the location of the chip level, i.e., the chip height, in the digester, the chip level can easily deviate from desired or optimum operating range.
In order to adjust and control the chip level, the operator must have some means for determining the chip height at any given time. The use of sight holes or other visual devices are impractical since pulping digesters are pressure vessels and are normally operated at pressures on the order of 165 psig. Several different devices have been used in an attempt to provide the operator at least with a relative indication of when the chip level goes above or recedes below a preferred operating height. One such device is a torque tube driven by an electric motor that is inserted into the top portion of the digester. The torque tube carries two radial paddles that are situated at a position in the digester that is somewhat above the desired operating height for the chip level. A device for measuring the angular displacement of the paddles relative to a given location on the torque tube is associated with the paddles and tube. When the paddles are rotating in the liquor alone, little or no angular displacement of the paddles occurs. However, when the chip level rises above the location of the paddles, a substantial increase in the angular displacement of the paddles is effected. When the angular displacement exceeds a predetermined amount, the chip level presumably has risen above the location of the rotating paddles. When the operator makes the appropriate adjustment to lower the chip level, he can readjust the digester throughput to a steady state condition once the angular displacement of the paddles drops to its normal level, since presumably at that time the paddles are again rotating only in the liquor. Although this device is sometimes effective to provide an indication whether or not the chip level is above or below a given height in the digester, the device cannot provide a relative measurement of chip height. Moreover, this device is necessarily mechanical in nature and is subject to high maintenance and to down time during which the operator of the digester has no convenient means of monitoring the chip level. In addition, this device tends to be unreliable, as new chips entering the digester pass through the paddles and cause transient increases in the angular displacement of the paddles that is not necessarily indicative of a change in the chip height.
To solve the problems associated with prior art devices such as those described above, a broad object of the present invention is to provide a means for determining chip level in a pulping digester than can provide a direct indication of the chip height as opposed to a relative indication of whether the chip level is above or below a given location in the digester. Further objects of the present invention are to provide a means for so doing that provides a signal that is easily converted to information readily understandable by a digester operator; to provide such a means that has no moving parts positioned in the digester, and has, therefore, a greater reliability and is easier to maintain and install than prior art devices; and to provide such a means that will yield repeatable results and will so function over extended periods of digester operation.